const Boxes = document.getElementById('boxes')
const btn = document.getElementById('btn')
btn.addEventListener('click', () => Boxes.classList.toggle('big'))
function createbox() {
    for (let i = 0; i < 6; i++) {
        for (let j = 0; j < 6; j++) {
            const newbox = document.createElement('div')
            newbox.classList.add('box')
            newbox.style.backgroundPosition = `${-j * 100}px ${-i * 100}px`
            Boxes.appendChild(newbox);
        }
    }
}
createbox();